1. Field of the Invention
The present invention relates to wireless communication systems and, more particularly, to a digital, cellular or microcellular radio system capable of providing voice and data communications to users via portable equipment.
2. Description of the Prior Art
In the field of radio communication, there has been an increasing demand for portable communication systems. Responsive to this demand, a variety of portable systems have been developed, such as cordless telephones, mobile radio telephones and radio paging systems. Although such devices have served the purpose, they have not proved entirely satisfactory under all conditions of service for a variety of reasons. For example, most portable systems are designed to only operate efficiently in a spectrum free of interfering signals. Consequently, the user or provider of such systems must apply to a governmental authority for allocation of an operating frequency, a process that is usually protracted. Also, for frequencies below 3 GHz, the spectrum is virtually fully allocated in most regions of the world. Since frequency allocations are generally made on a nation-by-nation basis, the allocation process is even more complex and protracted for those wishing to provide universal portable communication systems.
Another drawback of prior art portable systems is their lack of privacy. Most existing systems employ analog signal transmissions, making them vulnerable to easy eavesdropping with the use of scanners. Such analog systems are also subject to problems with false usage, accidental dialing, and the like.
To obviate the privacy problems, digital portable communication systems have been proposed. However, most digital systems are narrow band and must employ complicated voice coding techniques to reduce the bandwidth of the transmitted voice signal. These techniques normally increase the transmit delay which leads to a need for echo cancelling and reduces perceived audio quality.
A further disadvantage of conventional cellular systems is their hand off procedures, which are usually complicated to administer and often fail, leading to a loss of calls in progress. Even when working properly, most cellular systems have a noticeable gap in communications during hand off.
Although there has been a long recognized need for an economical, simple and efficient wireless integrated voice-data communication system capable of dependable operation in the presence of interfering signals, no practical system for doing so has yet been devised. Ideally, such a system would provide quality operation even in the presence of interfering signals; would economically employ fixed communication resources; would include mobile units that are relatively simple in construction and operation; would provide a high degree of security, preventing such problems as eavesdropping, false usage, accidental dialing and the like; would operate with low transmit delay to prevent signal degradation and reduced audio quality due to echo effects from signal reflections; would provide hand off which is not audible to the user; would allow operation as a data transmission system as well as a voice system; and would degrade gracefully in the presence of interference. The present invention fulfills this need.